Silver alloys

Dr. Dmitri Kopeliovich

• Silver
• Jewelry and tableware silver alloys
• Silver brazing alloys
• Coin silver
• Silver alloys for electrical contacts
• Chemical compositions of some silver alloys
• Properties of some silver alloys

Silver (Ag) is a transition metal with an atomic number 47.
The atomic mass of silver is 107.8682(2). The most abundant and stable isotopes of silver are ¹⁰⁷Ag and ¹⁰⁹Ag).
A nucleus of ¹⁰⁷Ag is composed of 47 protons and 60 neutrons.

Silver is found in chemically uncombined state: as pure silver (native silver) or in form of a silver-gold alloy (electrum)).
Silver occurs also in a combined form as Ag₂S (silver sulfide), AgCl (silver chloride), Ag₃SbS₃ (silver sulfantimonide).
The most prominent properties of silver:

• Electrical conductivity. Silver also has the highest electrical conductivity of all elements.
• Thermal conductivity. Silver has the highest thermal conductivity (429 W/m*K) of all Metals.
• Chemical inertness. Silver does not oxidize in clean air and clean water at normal temperatures. Silver reacts with sulfur compounds, halogens and ozone. Silver can be dissolved in nitric acid (HNO₃)and in hot concentrated sulfuric acid (H₂SO₄). Due to its chemical stability platinum is referred to noble metals together with ruthenium, rhodium, palladium, platinum, osmium, iridium and gold.
• Malleability and ductility. Silver has an excellent ability to be compressed (malleable) or stretched (ductile).

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Pure (fine) silver is very soft (26 HV in annealed state) therefore it is rarely used in an non-alloyed state.
The strength, modulus of elasticity and hardness of silver may be considerably increased by alloying with other metals.

The most popular jewelry/tableware silver alloy is alloyed with 7.5% of copper. The alloy is called Sterling silver.
Sterling silver has a hardness 71 HV in annealed state. The hardness may be increased to 140 HV by strain hardening (cold work).
Jewelry and tableware pieces made of sterling silver has good wear and scratch resistances. On the other hand the alloy is very malleable and ductile what makes it easy to fabricate.

Tarnish resistance of sterling silver may be improved by replacing of 0.5-1% of copper with germanium (Ge) (Argentium sterling silver).

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Brazing is a method of joining two metal work pieces by means of a filler material at a temperature above its melting point but below the melting point of either of the materials being joined.
Flow of the molten filler material into the gap between the work pieces is driven by the capillary force. The filler material cools down and solidifies forming a strong metallurgical joint, which is usually stronger than the parent (work piece) materials. The parent materials are not fused in the process.

Silver alloys are popular filler material used for brazing Copper alloys, Nickel alloys, Stainless steels, Carbon steels and Alloy steels.
The melting point of general purpose silver brazing alloys is in the range 1200-1500ºF (649-816ºC).
Besides silver the alloys contain copper 20-50% of (Cu) and 10-40% of zinc (Zn). Some of the alloys may also contain 2-5% of tin (Sn) and/or 0.1-0.25% of silicon (Si) for better flowing ability and aesthetic appearance.

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Gold and silver have been used as coinage metals for thousands years.
Content of silver in different coin silver alloys varies within 50-99.99%.
Other metals contained in silver coin alloys are copper and zinc.
One of the standard coin alloys is Britannia silver with the composition 95.84% silver and 4.16% copper. Britannia silver was introduced by English Parliament in 1697 as a replacement to sterling silver.
The modern Silver Britannia coins (since 2013) and American Silver Eagle (the official silver bullion coin) contain 99.90% silver.

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Exceptional electrical and thermal conductivities combined with chemical stability make silver very useful for electrical applications.
Silver alloys are widely used in various break contacts, relays, switches, sliding and rolling contacts.

The alloys and pseudo alloys (Metal Matrix Composites) are prepared by casting and sintering technologies.

The most popular silver alloys for electrical applications:

• Fine silver. This is pure metal containing at least 99.9% silver. Fine silver is used when highest electrical and thermal conductivity of the contacts are required.
• Silver-copper alloys containing 10-25% of copper are most popular for electrical applications. Ag-Cu alloys combine very good electrical conductivity with good wear resistance and increased hardness.
• Silver-graphite alloys are composite materials containing up to 5% of Graphite. The materials are fabricated by sintering method. Silver-graphite alloys are used for various sliding contacts where good tribological properties (low coefficient of friction and good seizure resistance) are important (e.g. brushes and other sliding contacts).
• Silver-nickel alloys have high strength and wear resistance. Nickel content in the alloys is normally 5-10% but there are also alloys containing up to 50% nickel.

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(Materials Data)

• Fine silver
• Sterling silver
• Silver-ABA

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• Steels and cast irons
• Aluminum alloys
• Copper alloys
• Magnesium alloys
• Nickel alloys
• Titanium alloys
• Gold alloys
• Platinum alloys

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